Study on a Unified Fracture Mechanics Parameter Controlling Crack Growth Behavior and its Evaluation Method

提出了将裂纹能量密度作为一个参数，以统一的形式描述各种形式的裂纹行为，无论断裂类型是脆-韧性断裂、疲劳断裂还是蠕变断裂。在本研究项目中，研究了用裂纹能量密度表示裂纹扩展速率的方法和通过数值分析估算这些参数的方法，并利用最新研制的观察裂纹尖端变形的装置，研究了基于裂纹能量密度的蠕变裂纹扩展的起裂条件。结果表明：1.从裂纹能量密度的角度考虑延性裂纹、疲劳裂纹和蠕变裂纹的扩展速率，可以用单位时间(疲劳情况下为一周)的裂纹能量密度增量和单位时间内的断裂阻力增量和裂纹能量密度分布的梯度来统一描述它们的扩展速率。将非连续裂纹模型扩展为能够计算裂纹能量密度分布和裂纹尖端数值的模型，使之能够处理蠕变问题，并发展了一种用路径无关积分计算该模型中裂纹能量密度的新方法。通过对一个标准问题的分析，验证了该模型的有效性。研制了观察炉外裂纹尖端变形的装置，并进行了蠕变裂纹扩展实验。由上述方法计算出的裂纹开始扩展时的裂纹能量密度值表明，存在材料特有的裂纹能量密度临界值。

The crack energy density was proposed as a parameter which may describe all kinds of crack behavior in a unified form regardless of fracture types such as brittle-ductile fracture,fatigue fracture and creep fracture. In this research project,the way of expressing crack growth rate by the crack energy density and the evaluation method of thes parameter by numerical analysis are studied and,moreover,the initiation condition of creep crack growth is studied based on the crack energy density by using a newly-developed equipment for observing the deformation around a crack tip. The results are as follows;1. By considering the crack growth rates of ductile,fatigue and creep cracks from the standpoint of crack energy density,it is made clear that their growth rates can be described in a unified form by the increments of crack energy density at crack tip and fracture resistance expressed by crack energy density per unit time(per one cycle in case of fatigue) and the gradient of crack energy density distribution.2. The discontinuous crack model proposed as a model,which enablesus to evaluate the distribution of crack energy density as well as the value at crack tip,is extended so as to be able to deal with a creep problem and a new method to evaluate the crack energy density in this model by a path-independent integral is developed. The availability of the model is confirmed through the analyses of a standard problem.3. An equipment for observing the deformation around a crack tip out of the furnace is developed and the experiment of creep crack growth is carried out. The value of crack energy density evaluated by the method above at the time when a crack starts to grow suggests the existence of the critical value of crack energy density peculiar to the material.